Nanoscale hybrid Silica/Polymer Janus Particles with a double-responsive

Hybrid nanoparticles combining relevant materials properties from different material classes will be one of the most important building blocks for multifunctional materials in future technologies.⁷

In this project all knowledge from the previous projects about hybrid core-shell-corona particles and the modification of the intermediate silica shell was used to develop a versa-tile large-scale synthesis strategy for dual-responsive hybrid Janus nanoparticles with a silica core and a hemispherically attached PDMAEMA-corona in a size range of 100 nm.^9,10

Scheme 2-4. Synthesis of SiO2/PDMAEMA hybrid Janus nanoparticles.

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The synthesis was based on a modified version of the Pickering emulsion technique in combination with surface initiated atom transfer radical polymerization (ATRP) in a

“grafting from” approach (Scheme 2-4). In a first step, colloidal stability was provided to latex particles prepared via emulsion polymerization using 30 nm silica nanoparticles that adhere to the surface of the growing poly(vinyl acetate) (PVAc) droplets. This resulted in polymer latexes which are armored with a layer of tightly immobilized nanoparticles. All silica particles were uniformly embedded in the surface of the PVAc phase. On basis of SEM pictures of the resulting Janus particles it can be estimated that ¾ of the particle surface is embedded and ¼ was unprotected and thus, free for modification (Figure 2-16).

Figure 2-16. (A) cryo-SEM and (B) TEM images of PVAc latexes armored with SiO2 nanoparticles ob-tained by Pickering emulsion polymerization.

To activate the surface of the silica particles for the growth of the final stimuli-responsive corona, the exposed side of the silica particles was modified with a silane-carrying ATRP initiator, (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane. The structural changes during the experimental procedure were monitored via FT-IR and DLS.

After isolation of the hemispherically functionalized SiO2 nanoparticles, ATRP was used to grow a PDMAEMA hemicorona by a “grafting from” polymerization of 2-(dimethylamino)ethyl methacrylate) (DMAEMA), yielding well-defined stimuli–

responsive Janus nanoparticles. During this polymerization, the reaction can be conven-iently quenched at different reaction times to furnish hemicoronas with different lengths of PDMAEMA chains (Figure 2-17 A). With increasing the molecular weight from 55 kg/mol to 108 kg/mol the hydrodynamic radii increased, too. Thus, the dimensions of our

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hybrid particles can be controlled via the length of the PDMAEMA chains in the corona (Figure 2-17 B).

Figure 2-17. (A) GPC traces of cleaved PDMAEMA polymers with different molecular weights Mn. (B) Intensity-weighted hydrodynamic radii distribution (DLS) of (A) pristine silica particles and (B/C/D) Janus SiO2/PDMAEMA nanoparticles with different molecular weights Mn (kg/mol) according to GPC: (B) 55, (C) 87, (D) 110. All samples are measured at pH 8 and RT.

Further, TEM and SEM images in Figure 2-18 allowed the detailed investigation of the Janus character of the obtained particles. The Janus nanoparticles are well-defined with the two components clearly separated with a sharp interface between the SiO2 particles on one side and the grafted PDMAEMA on the other side.

Figure 2-18. Janus SiO2/PDMAEMA nanoparticles. (A) Representative TEM image of hybrid SiO2/PDMAEMA Janus nanoparticles. (B) SEM images of Janus nanoparticles with clearly separated com-ponents (SiO2 particle and PDMAEMA corona).

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DLS and FT-IR demonstrated the successful polymerization of a PDMAEMA corona and in combination with TEM and SEM images a comprehensive proof was given for a suc-cessful immobilization of a PDMAEMA hemicorona on the activated SiO2-particles and a successful synthesis of Janus nanoparticles.

In the last section, the pH- and temperature-responsive behavior of the Janus nanoparti-cles was highlighted via DLS and turbidimetry measurements (Figure 2-19). First, DLS was used to investigate the dual-responsive behavior of PDMAEMA at 20 °C (Figure 2-19 A). The hydrodynamic radius distributions of one sample were studied at different pH values. The particles displayed a clear increase of the radius with decreasing pH, due to the stretching of the polymer chains with increasing protonation.

Turbidity measurements were conducted to demonstrate the pH-dependent LCST behav-ior of the Janus NPs (Figure 2-19 B). The coil-to-globule transitions at the cloud point of the grafted particles are sharp and the clout points decrease from 85 °C at pH 7 to 30 °C at pH 10.

Figure 2-19. (A) Intensity-weighted hydrodynamic radii distribution (DLS) of Janus nanoparticles at pH 7, 8 and 10 at RT. (B) Turbidity measurements of the Janus SiO2/PDMAEMA nanoparticles (c = 0.1 g/L) at different pH: pH 10 (black,■), pH 8 (red,●) and pH 7 (blue,Δ).

An unexpected, concentration dependent self-assembly and clustering of the Janus nano-particles at low pH values (˂ 5) was observed in the TEM images in Figure 2-20. In more concentrated solutions we find a pH-dependent aggregation into linear strings. At low pH (pH 4) the PDMAEMA chains are charged and highly stretched. At low particle concen-tration only isolated particles can be observed, but increased concenconcen-trations lead to the formation of short worm-like assemblies. In contrast, at high pH (pH 11) the PDMAEMA

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chains are collapsed. Then only isolated particles are found independent of the particle concentration. Further DLS measurements at different pH values from 0.1 g/L to 2 g/L confirmed the results obtained via TEM and the proposed mechanism of aggregate for-mation.

Figure 2-20. TEM images showing a concentration dependent clustering of the Janus SiO2/PDMAEMA nanoparticles at pH 4 (top) and pH 11 (bottom) at different concentrations (0.5 g/L, 1 g/L, 2 g/L) and sche-matic representation of the Janus nanoparticles and their self-assembled structures.

In conclusion, an efficient and simple strategy based on a modified Pickering emulsion technique was developed for a large scale synthesis of well-defined and high-quality hy-brid Janus nanoparticles with a 30 nm SiO2 core and a temperature- and pH-responsive PDMAEMA hemicorona in the range of 100 nm. Most importantly, this synthetic ap-proach is easily scalable and can be amended to furnish a wide range of nanoscale hybrid Janus particles with a wide variety of different stimuli-responsive polymers.

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2.6 References

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